To enhance the understanding of the discussion that follows, the abbreviations and terms listed below will have the definitions as shown.
ADC—Analog to digital converter
BEOL—Back end of line
CA—Tungsten contact between metal and polysilicon
Csub—Adjustable capacitor
DAC—Digital to analog converter
FEOL—Front end of line
MIMCAP—Metal-insulator—metal capacitor
MOS—Metal oxide silicon
RF—Radio frequency
VNCAP—Vertical native capacitor
On-chip capacitors are critical components of integrated circuits that are fabricated on silicon semiconductors. These capacitors are used for a variety of purposes including bypass and capacitive matching as well as coupling and decoupling. These various uses of capacitors are generally well known and need not be elaborated upon.
One of the drawbacks in present on-chip capacitor designs is the inability to accurately predict the value of an on-chip capacitor. In fact, an error of 5% or more in the design value of the capacitor is common, resulting in differences of 10% or more between high and low capacitor values. These variations are based on factors, such as variables in silicon technology manufacturing process values and temperatures, and are particularly significant in the production of VNCAP and MIMCAP on-chip capacitors. Unfortunately, there is no known procedure to adjust or tune the capacitance value of a capacitor after its manufacture.
Precision on-chip capacitors are particularly important for use in integrated circuits requiring accuracy. Examples of such uses are radio frequency matching circuits and analog circuits, such as DAC, ADC and switch capacitor filters with precisely adjustable capacitance. The need for accuracy results in high rejection rates and decreased yields in chip production.